The present invention relates to heat exchangers for high temperature and high pressure service, in which the temperature gradient in a thick tube sheet is decreased and thermal stress caused thereby is reduced accordingly. More particularly the present invention relates to heat exchangers, in which a hot fluid before being heat exchanged is prevented from directly contacting a thick tube sheet, said hot fluid being introduced from a separate section through a group of tubes into a heat exchanger shell and discharged through a fluid outlet nozzle opened at the tube sheet, with groups of those tubes for fluid before and after heat exchange being alternately arranged, thereby resulting in a large reduction of thermal stress in the thick tube sheet.
Generally, heat exchangers are used to recover heat from or exchange heat with a hot gas generated by burning, or by a chemical reaction or the like in chemical and various other industrial plants.
Conventionally, various types of heat exchangers are used, one of which uses a U-tube type heat exchanger. The heat exchanger of this type is superior to others in that thermal stress is prevented from occurring, which is caused by different thermal expansion being induced by the temperature difference between the tubes and the shell.
The heat exchangers of the U-tube type being conventionally used are shown in FIGS. 1 (a), (a'), (b) and (b'). FIG. 1 (a) is a schematic section of a an example of conventional U-tube heat exchangers, wherein U-tubes 2 are arranged in a shell 1 having inlet and outlet nozzles for a first fluid, a tube sheet being secured to the bottom end of the shell 1, the ends 2a, 2b of the U-tubes passing through and being secured to the tube sheet 3, opened to the outside of the shell. On the side of the tube sheet 3 opposite to the shell 1 there is provided a channel enclosed by a stationary head 4 and a chamber cover 5, the chamber being divided into two volumes by a pass partition 6. One of the two volumes is provided with an inlet nozzle 7 for the second fluid and the other room is provided with an outlet nozzle 8 for a second fluid, the ends 2a of the U-tubes 2 to admit the second fluid being completely opened to one volume and the other ends 2b of the U-tubes to discharge the second fluid being completely opened to the other volume. FIG. 1 (a') shows the section along line A--A in FIG. 1 (a).
FIG. 1 (b) is a schematic section of another example of conventional U-tube type heat exchangers. This type comprises, similarly to the one in FIG. 1 (a), a shell 1 in which U-tubes 2 are contained, and a main chamber enclosed by a stationary head 4 and a main chamber cover 5. In the main chamber an inner chamber 9 having an inlet nozzle 7 for the second fluid is provided, the ends 2a of U-tubes to admit the second fluid being opened at the tube sheet of the inner chamber 9 and the other ends of U-tubes to discharge the second fluid being opened in the annular portion of the tube sheet between the inner chamber 9 and the stationary head 4. FIG. 1 (b') shows the section along line B--B in FIG. 1 (b).
In the operation of the type of heat exchanger shown in FIG. 1 (a), a hot second fluid enters through the inlet nozzle 7 for the second fluid into the chamber and flows further through the ends 2a of U-tubes into the U-tubes 2, and after exchanging heat with the first fluid in the shell 1, flows, through the chamber, out from the outlet nozzle 8. Since the chamber is divided into two volumes by the pass partition 6, the chamber on the inlet side of the second fluid is filled with the hot second fluid, making the tube sheet hot.
The chamber on the outlet side of the second fluid is filled with the cold second fluid, making that portion of the tube sheet 3 colder than the inlet side. The temperature distribution in the tube sheet 3 becomes asymmetric as shown in FIG. 1 (a"), inducing a large thermal stress and causes the designing of the heat exchanger difficult.
In the operation of the type of heat exchanger shown in FIG. 1 (b), a hot second fluid enters from the inlet nozzle 7 into the inner chamber 9, flows through the inlet ports 2a of the U-tubes into the U-tubes 2, and after exchanging heat with the first fluid in the shell 1 and being cooled, flows out from the outlet ports 2b of the U-tubes into the annular space surrounding the inner chamber 9, and then leaves the heat exchanger through the outlet nozzle 8. In this case, the inside of the inner chamber 9 is filled with the hot second fluid, so the tube sheet 3 contacting the hot fluid becomes hot, but on the other hand, the tube sheet portion outside the inner chamber 9 contacts the cold second fluid after heat exchange and is made cold and therefore, the temperature distribution in the tube sheet is made as shown in FIG. 1 (b"), the central portion being high and the peripheral portions being low. This difference in the temperature induces thermal stress in the tube sheet 3. The stress in this case is a little smaller than the case in FIG. 1 (a), but still it is difficult to determine the arrangement of U-tubes for the case in FIG. 1 (b).